Indirectly heated cathode of increased efficiency



Dem 1955 J. w. BUSBY ET AL 2,726,346

INDIRECTLY HEATED CATHODE OF INCREASED EFFICIENCY Filed Jan. 25, 1952 v[0 29 M 5 y /Za 1/ 5 in j/W 1a f /Z\ 1 1/ jj l if; L S, 'f V If 4/ ZSatent 2,726,346 Patented -Dec. 6, 19.55

1 N DIRECTLY HEATED CATHODE 1 OF INCREASED EFFICIENCY ApplicationJanuary25, @1952, Serial No..268,322 '6 Claims. (Cl. 31337) This inventionrelates to a cathode assembly, including a cathodeof the indirectlyheated type, of increased efiiciency, and more particularly. to a ruggedcathode assembly. characterized by reduced. heat losses.

In sometypes of electrontubes, such asthe pencil type, the cathode isspaced appreciably from the header or stem through which heater leadsextend. This appreciable spacing has rendered it necessary to use heaterlegs integral with the heater of considerable length for connecting theheater to the heater leads referred to. The heater structure employed inthe pencil type tube aforementioned includes a double coil-which isreceived within thesleeve, the legs of the. heater extending outside ofthe sleeve. :-Since.the-heaterlegs are integral with the double coil andhave the same diameter raszthe .coil wire, they .become heatedtosubstantially the same .degreeas thecoil and dissipate anv appreciableamount of heat to the region .surroundingthem. The heat dissipated isnot only the heat .directly generated in .the heater legs ;but also heatxtransferredzby conduction by the heater .proper:to TIh legs. Suchtransfer is likely to-occur-because-of,the higher temperature of the,heater proper, resulting from the greater heater concentration inthe-heateraproper than ;in ithe'glegs, and the shielding efiect provided;-by the cathode sleeve, which is absent, from the region-of theheaterlegs.

.This heat dissipation by the cathode sleeve and heater consumesv anappreciable part of-the power intended to-energize theheater. Because ofthis, either the-heat transferfrom theheaterto thesleeve isinadequate'for a desired emission, or added, power to-theheater is necessary tomakeup for the-heat dissipation referred to.

.In additionto undesirable heat dissipation, the relatively. long heaterlegs heretofore employed in the fpencil type tube have contributed to.lossof ruggedness ofthe cathode assembly. The heater coil received bythe cathode sleeve relies in parton a snug fit in the sleeve and in partonthe heater legs for support. Where the heater legs are relatively longand of relativelyrsmall cross-section, as is the'case when theyareintegral with the heater coil, theyprovide a relatively Weak, supportfor the heater coil. This. supportmay-be inadequate, and impactsincidental -to normal use of a tube in which theassembly isused,-may--cause the heatercoil tobecome displaced from the sleeve,thereby impairingfnrther utilityof the tube.

The, lack of ruggedness of a support providedby relatively long heaterlegs integral with the heater, is alsoa disadvantage when mounting theheater in the :cathode sleeve. -Some force is usually involved ininserting a heater in. a, sleeve, especiallywhen the transversedimensions .of a. heater structure are. such as,to providea relativelysnug fitof theheater in the'sleeve. Sincethis'force is transmittedthrough the heater legs,.collapse or: buckling of theheaterflegssometimes occurs during amounting operation.

Accordingly, itis an object of the inventiontoprovide an improvedcathode assembly of the indirectly heated yP ,A further object is toprovide an indirectly heated cathode of increased efficiency.

Another object is to provide an indirectly heated cathodeassembly.wherein undesired heat losses by the cathode sleeve and heater, isreduced.

A furtherobject is to provide an indirectly heated cathode assemblyincluding an elongated heat isolating ,sup- .port forthe cathodesleeve-of such assembly resultingrin increased spacing of thesleeve;from lead-ins serving the heater of the assembly, and leads ofnovel construction between the heater .and said lead-ins for reducing-heat losses-therefrom.

Another object is to providea tubular heatisolating cathode sleeve.support having a falling characteristieof heat conductivity atincreasingtemperatures thereofand leads within said support'having relativelyhighheatradiating properties forraising the temperature of thesupport tothereby reduce its heat conductivity, whereby heat losscsjfroin saidsleeve are reduced.

A further object is to provide an indirectly heated cathode assembly ofincreased ruggedness.

Briefiy considered, the indirectly heated cathode assembly or" theinvention includes a tubular support for the cathode sleeve, havingreduced heat conductivity and characterized by appreciably less heatconductivity in relation-to applied heat, at relatively hightemperatures than atrelativelylow temperatures. In combination with thistubular-support the invention provides for a heater disposed within thecathode sleeve andhaving relatively shortintegral legs, and tubularleads having relatively large area surfaces connecting the short heaterlegs..to heater lead-ins. The tubular leads extend with-in -thetubular'support and are substantially coextensive 'longitudinallytherewith.

This combination is of appreciable advantage. *The tubular structure ofthe leads provides-relatively large heat radiating surfaces. Therefore,whatever heat is -generated in the leads is readilyradiated from theirrelatively large area surfaces, to the inner walls of'the tubularsupport. The temperature of the tubular cathode support is'therefore.increased as a result of such radiation,-which serves to decrease itscharacteristic of heat conductivity. -While one advantage of thetubularleads resides-in theirgreater heat radiation, as aforementioned,further advantages 'of the tubular structure of the leads compriseincreased ruggedness of support provided thereby, increased electricalconductivity thereby reducing heat generation therein as compared toconventional'heatcr legs,and reduced heat conductivity as compared toconventional solid lead-ins. 'A tubular structure has a favorablecharacteristic of stiifness over a solid structure ofequal diameter.-Thisstiffness enables the leads to support firmly in a cathode sleeve aheater to which theyare connected and facilitates insertion of theheater into a cathode sleeve. The combination of the-cathode sleevetubular support ahd-the-tubular leads for the heater, therefore,substantially reduces-heat dissipation both from thecathode sleeve asWell-as from the heater leads, thus permitting reduced heater-powerinput without sacrifice of emission. Furthermore, the-tubular leadsreferred to provide the desired support of the heater, particularly whensuch support is called uponto traverse an appreciable distance as isinvolved in a pencil type tubestructure utilizing'atubular support forthermal isolation of the sleeve, as aforementioned. Further objects andadvantages of the invention .will become apparent'as the presentdescription proceeds.

Referring to-the drawing, in connection witha more detailed-descriptionof the invention,

Figure lshows a sectional elevationof a conventional -penciltypeelectron'tube; Figure 2 is a view in sectional elevation of thecathode assembly of a pencil type tube wherein the cathode sleeveissupporte'd on a sleeve made of a material of low heat-conductivity forreducing heat dissipation from the oVer the relatively long sleeve byconduction and wherein the structure requires relatively long heaterlegs from lead-ins to the heater proper; and

Figure 3 is a longitudinal sectional view of a cathode assemblyaccording to the invention useful in a pencil type electron tube andwherein tubular leads are employed for connecting the heater lead-ins tothe heater distance separating the heater and the lead-in as aconsequence of the use of a low heat conducting support for the cathodesleeve.

The pencil type of electron tube, as shown in Figure 1, includesopposite end sections 1 0 11 between which is mounted a disc 12. Thedisc 12 supports a grid 12a and is electrically insulated from the endsections referred to by means of glass tubes 13, 14. In section 1 0 ismounted a. cylindrical anode 15 and the section is closed at its freeend by exhaust tubulation 16.

Section 11 comprises a cathode assembly, and includes an outer tubularmember 17 serving as a cathode lead-in. Within member 17 is fixed atubular support 18 for the coated cathode sleeve 19. A heater 20 havinglegs 21, 22 is connected to lead-ins 23, 24 which in turn are adapted tobe connected to a suitable power source, not shown, for raising thecathode sleeve to an electron emitting temperature. The lead-ins 23, 24are held in spaced relation by an insulating spacer disc 25. Alsoincluded in the cathode assembly is a getter 26 mounted across leadins27, 28, the getter being flashed by resistance heating thereof. Thelead-ins 23, 24, 27, 28, are sealed through glass stem 29 which servesto close the free end of section 1] of the tube.

The foregoing tube structure is subject to several disadvantages. Thesleeve support 18 is usually made of a metal having relatively high heatconductivity. Therefore, an appreciable amount of heat is conducted byit away from the cathode sleeve 19. Furthermore, the heater legs 21, 22,being integral with the heater 20 and of the same diameter as the heaterwire, become heated during energization of the heater. The heat sogenerated in the legs referred to, serves no useful purpose and on thecontrary, since it is dissipated by radiation, constitutes an undesiredheat drain. To compensate for the heat lost by the sleeve 18 and legs21, 22, as aforementioned, it is necessary to provide an increased powerinput to the heater 20. In addition, the structure, as shown in Figure1, provides a relatively weak support for the heater. In thisconnection, it may be noted that the heater 20 and the legs 21, 22thereof are usually made of wire having a diameter of only 4 mils. Thelegs therefore, are readily yieldable, not only during the operation ofinserting the heater into the cathode sleeve during manufacture of thetube, but also in response to stresses tending to displace the heaterfrom the sleeve during operation.

In attempting to remove the aforementioned difficulties, applicantsinterposed a tubular support 30, shown in Figure 2, between the cathodesleeve 19, and support 18, the tubular support having reducedconductivity. By way of example, the tubular support 30 may be made ofan alloy known commercially as Kovar, and including nickel, iron andcobalt. This alloy is characterized by a relatively low order of heatconductivity. To further lessen heat conductivity by the support 30, itswalls may be reduced to a foil thickness of 0.0005 inch.

However, several objections were found to characterize the improvedstructure shown in Figure 2. The interposition of the tubular or sleevesupport 30 between the cathode sleeve and support 18 involved anappreciable increase in the spacing between the heater 20 and theleadins 23, 24. To traverse this increased spacing, the heater legs 21,22 were made substantially longer than in the structure shown in Figurel. This increased length of the heater legs increased the electricalresistance and consequently the temperature thereof to an appreciabledegree resulting in severe losses in heater power. While the hot legsradiated some heat to the sleeve support, the

effect of such heat in reducing the heat conductivity of the support,was negligible in respect of the efiiciency of the cathode assembly.

To fully solve the problem of undesired heat loss from the cathodeassembly, applicants found it necessary to resort to a novel combinationof the thermal isolation sleeve 30 with a novel lead structureconnecting the legs of the heater to their associated lead-ins. As shownin Figure 3, a cathode assembly is provided according to the inventionin which the heat insulating sleeve 30, which may be similar to diesleeve 30 shown in Figure 2, is interposed between cathode sleeve 19 andsupport 18. The heater legs 21, 22 are appreciably shorter than inFigures 1 and 2 and are connected to lead-ins 23, 24 by means of tubularleads 31, 32. The tubular leads may have an outer diameter of 10 milsand walls 2 mils thick. In one example, the tubular leads 31, 32 were 11mm. long. One or both ends of the tubular leads may be flattened forwelding or brazing to the heater legs 21, 22 and to the lead-ins 23, 24.Preferably the tubular leads referred to may have a coating ofinsulation thereon to prevent shorts.

The tubular leads 31, 32 may be made of any suitable material.Preferably they are made of a refractory metal such as tungsten,molybdenum or tantalum having relatively-high melting pointtemperatures. This is because relatively high temperatures are usuallyused in processing the cathode after mounting in a tube. If relativelylow temperatures should be feasible during such processing, othermaterials, such as nickel, may be employed for the tubular lead-insreferred to.

The tubular leads 31, 32 coact with the thermal isolation sleeve support30 to increase the efiiciency of a tube in which they are used, bydecreasing appreciably undesired heat dissipation by the cathodeassembly, thereby rendering a reduced power input to the heater 20adequate for good operation of the tube. The increased cross-sectionalarea of the tubular leads over that of the heater legs serves todecrease their electrical resistance to thereby conserve the heaterpower. Whatever heat is generated in the leads is effectively radiatedby their relatively large area surfaces to the support 30. As aconsequence, sleeve support 30 is heated, and since its heatconductivity characteristic becomes less at higher temperatures, itsutility as a thermal isolation means for the cathode sleeve 19 isincreased. Therefore, the tubular leads 31, 32 not only contribute toincreased efliciency of a tube in which they are used, by virtue oftheir increased conductivity, but also because they increase theeffectiveness of the thermal isolation sleeve 30 in reducing heatconduction from the cathode sleeve.

Apart from their aforementioned coaction with the heat isolation shield30 to improve the efficiency of a tube, the tubular leads 31, 32, haveindividual advantages. One of these advantages resides in a contributionof ruggedness to the cathode assembly. This is important where theheater is spaced appreciably from its lead-ins. Thus, the tubular leadsprovide a relatively rugged support for the heater 20 as a consequenceof which it is restrained from displacement from the sleeve 19.Furthermore, the tubular supports provide a relatively stifi heaterassembly which is of advantage when threading the heater into thecathode sleeve. In addition, the tubular construction of'the leads,renders a flattening thereof at one or both ends, relatively easy, forwelding or brazing to the heater legs and leads 23, 24. In oneembodiment of the invention, only one end of the tubular leads isflattened for welding to the leads 23, 24, the other ends being open forreceiving the free ends of the heater legs to facilitate fixing theheater legs to the tubular leads referred to. It will be noted from thedimensions previously given herein of the heater legs and tubular leads,that the heater legs are readily extendable into the end openings in thetubular leads. When the heater legs are once extended into the endopenings in the tubular leads, they are effectively held in positionwithout special holding means, while a welding or brazing operationtakes place. In view of the foregoing advantage, this embodiment ispreferred in practicing the invention.

It will be noted from the foregoing, that one advantage of the tubularleads 31, 32, is their relatively large area cross-section, as a resultof which their electrical conductivity is increased. However, anincrease in the crosssectional area of the heater leads also increasestheir heat conductivity. Therefore, it is desirable to keep thecrosssectional area of the heater leads within prescribed bounds, evenif this should involve appreciable heat generation therein, if theadvantage of increased electrical conduc tivity is not to be negativedby the disadvantage of excessive heat conductivity. Applicants havefound that the tubular structure of the heater leads 31, 32 according tothe invention, contributes appreciably in realizing the aforementionedadvantage while reducing to tolerable limits the disadvantage of heatconductivity.

Thus, the relatively large radiating surfaces provided by the tubularstructure effectively dissipate by radiation to the sleeve support 30any heat generated therein, as a consequence of a reduced cross-sectionthereof desired for reduced heat conduction from the heater.

However, the cross-section of tubular leads 31, 32 according to theinvention is such that most of the heat radiated by the tubular leads31, 32 is heat conducted to them from the heater, the cross-sectionbeing such that a relatively high electrical conductivity results andreduced resistance losses take place.

Applicants have found that a heater assembly having a heat isolationsupport 30 of a wall thickness of about 0.05 mil, and tubular heaterleads having an outer diameter of about mils and a wall thickness ofabout 2 mils, is characterized by substantial increase in efficiencyover structures having either relatively long heater legs or conventional lead-ins of increased length for reaching relatively shortheater legs. In this connection, it may be mentioned that heaterlead-ins usually employed in the type of electron tube aforementionedhave a diameter of 16 mils and are solid in cross-section. A lead havingthis structure and connected directly to a heater would conduct aprohibitive amount of heat from the heater and in view of its smallerradiating surface in relation to its diameter, would contribute toreduced eificiency of the cathode assembly.

It will be apparent from the foregoing that the invention provides anelectron tube of increased efficiency. To this end the cathode assemblyof the tube is provided with a thermal isolation support for the cathodesleeve and the heater is provided with tubular leads coacting with thesupport referred to, to reduce heat losses from the assembly. Inaddition, the tubular structure of the leads contributes to ruggednessof the cathode assembly and facilitates manufacture of the assembly.These features of the invention contribute to an improved electron tube.

What is claimed is:

1. A cathode assembly for an electron tube comprising a cathode sleeve,a heater within said sleeve and having relatively short legs extendingfrom one end thereof, a support fixed to said one end of said sleeve, alead-in for said cathode sleeve connected to said support, lead-ins forsaid heater spaced from said heater legs and said support, and leadsconnecting said legs to said lead-ins, at least a portion of said leadsbeing adjacent said support, said support being made of a materialhaving a coefiicient of thermal conductivity that becomes smaller withincreases in temperature of said support, said leads being tubular forgood heat radiation, whereby appreciable heat transfer from said leadsto said support takes place for reducing the heat conductivity of saidsupport.

2. A cathode assembly comprising a cathode sleeve, a metallic supportfor said cathode sleeve made of an alloy of nickel, iron and cobalthaving reduced heat conductivity at elevated temperatures, a wire heaterin said cathode sleeve, lead-ins for said heater, said support beingdisposed between said heater and said lead-ins, and tubular leadsconnected to said lead-ins, said leads having a smaller cross-sectionthan said lead-ins and being connected to said heater, whereby saidleads are heated by resistance losses and by conduction from saidheater, said leads being relatively close to said support, whereby saidsupport is heated by said leads for reducing its heat conductivity andimproving the efiiciency of said assembly.-

3. A cathode assembly of increased efiiciency comprising a cathodesleeve, afirst elongated metallic support having one end engaging an endof said cathode sleeve, a second elongated metallic support having oneend engaging said first support adjacent to the opposite end of saidfirst support, a heater wire within said cathode sleeve, said heaterwire having-legs extending adjacent to said first support, and tubularmetallic leads of larger crosssectional area than said legs andextending adjacent to said second support, whereby said first'support isheated by said legs and said second support is free from heat from saidtubular leads, said first support being made of a metal having arelatively low coetficient of heat conductivity at relatively hightemperatures, said second support being made of a metal having arelatively high coefficient of heat conductivity at relatively hightemperature.

4. A cathode assembly for an electron tube comprising a cathode sleeve,a metallic support for said sleeve, said support having a relatively lowcoetficient of heat conductivity at relatively high temperatures andengaging one end of said sleeve, a heater within said sleeve, saidheater having two legs extending from said one end of the sleeve, twolead-ins spaced from said legs and adapted to be connected to a powersupply, and two tubular leads having relatively large radiating surfacesconnecting said legs to said lead-ins and being substantiallyco-extensive with and relatively close to said support, whereby heatgenerated in and conducted to said tubular leads is effectivelytransferred to said support, for improved efliciency of said assembly.

5. A cathode assembly for an electron tube comprising a cathode sleeve,a metallic support having relatively low coeificient of heatconductivity fixed to one end of said sleeve, whereby heat conductivityfrom said sleeve is reduced, a heater within said sleeve having legsextending from said one end thereof, lead-ins for said legs spaced fromthe legs, and cylindrical leads having a larger radius than said legsconnecting said legs to said lead-ins, said metallic support being madeof a material having a relatively low coefiicient of heat conductivityat relatively high temperatures, said leads being substantiallycoextensive with said metallic support and adjacent thereto foreffective heat transfer by radiation to said support, whereby saidsupport is heated by said leads for reduced heat conduction from saidsleeve.

6. In combination: a cathode sleeve; a tubular metallic support having arelatively low coefiiicient of heat conductivity at relatively hightemperatures engaging one end of said cathode sleeve; a heater withinsaid sleeve having legs extending from said one end thereof; solidelectrical conductors spaced from said legs; and tubular leads hav-References Cited in the file of this patent UNITED STATES PATENTS1,984,897 Rothe Dec. 18, 1934 2,200,954 Glassberg May 14, 1940 2,432,513Depew Dec. 16, 1947 2,441,224 Hector et a1 May 11, 1948 2,693,546 Sorget a1. Nov. 2, 1954

